The present invention relates generally to power amplifier systems, and more particularly to a method and apparatus to improve the linear performance of radio-frequency power amplifiers.
Communication services providers, such as cellular system operators, are subject to very strict bandwidth usage spectrum constraints imposed by the Federal Communications Commission (FCC). The FCC licenses transmission channels in the radio frequency spectrum and requires that signals be confined within certain emission limit masks in order to prevent interference caused by signals straying or spilling into adjacent transmission channels. The xe2x80x9cemission maskxe2x80x9d is a power spectrum density envelope. The maximum emitted power allowed varies as a function of the frequency offset from the nominal allocation center frequency. In other words, the emission mask determines the maximum power which may be emitted at a specific frequency for each frequency within the channel allocation. This requires that sideband spillover, the amount of energy outside the licensed channel, be sharply attenuated.
Meeting these emission mask requirements is specially difficult when implementing modern, digitally-based, modulation formats, such as Code Division Multiple Access (CDMA), or Time Division Multiple Access (TDMA). Attenuating the sidebands to meet FCC requirements using such modulation requires very linear signal processing systems and components. Additionally, these digital modulation formats generally require very high peak power in relation to average power levels. In almost every power amplifier design, some kind of linearity augmentation is utilized. Thus, designing linear components, and in particular power amplifiers, at radio frequencies is costly and challenging to achieve.
Radio-frequency (RF) linear power amplifiers (LPA), comprising a plurality of RF power transistors, are typically used in digital cellular base stations to boost the power of a transmitted signal. RF power amplifiers for cellular communications typically operate in the Megahertz (MHz) and Gigahertz (GHz) frequency regions. Boosting a transmitted signal usually requires a LPA with a high ratio of peak-to-average power output (dynamic headroom), typically of at least 10 dB. The challenge is to design LPAs which can provide such dynamic headroom while minimizing sideband spillover without distorting the boosted signal.
A typical cellular base station requires the overall gain of its LPA to be in the 45 to 60 decibel (dB) range. Some of this gain is achieved by designing an LPA with cascading RF power transistors, with each series of transistors known as a xe2x80x9cgain stagexe2x80x9d. A fundamental problem in designing linear RF power amplifiers is that power amplifiers are inherently non-linear devices and generate unwanted intermodulation distortion (IMD).
Linearity refers to a characteristic of power amplifiers where there is a substantially constant (linear) gain between an input signal and an output signal. Typically, power amplifiers only exhibit linear gain within a range of input signal voltage levels. This range is often called the linear region of the power amplifier. If the input signal voltage is below the minimum voltage for the linear region or above the maximum voltage for the linear region, then distortion of the signal occurs.
Another form of signal distortion, known as intermodulation distortion, manifests itself as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. IMD occurs when different frequencies from the input signal mix to produce sum and difference frequencies which did not exist in the input signal. It is the result of the behavior of amplifier components when operating outside the linear region.
Where a radio-frequency LPA is comprised of metal oxide semiconductor field effect transistors (MOSFET), it is commonly known that the transconductance of these devices exhibit a pinching off or limiting effect at moderate drain currents as opposed to MOSFET devices designed for high power switching and lower frequency applications. This is due to the fact that in order to achieve the high frequency performance necessary in RF applications, the channel width of each cell within the MOSFET must be substantially reduced. This channel width reduction causes the pinching off effect as the gate potential of the device is increased. This premature current limiting action produces a soft compression, which limits the power output linearity of MOSFET-based LPAs. Additionally, the transconductance prior to limiting is a non-linear function so the small signal gain of the device is strongly affected by the bias current.
Therefore, a RF linear power amplifier is desired which has substantially linear characteristics, minimizes both sideband spillover and intermodulation distortion, and does not increase the DC current consumption over conventional LPAs.